The annual event attracts big crowds to Llanwrtyd Wells
A former three-time winner of the world bog snorkelling championship has revealed his secrets as the event celebrated its 21st anniversary.
About 100 challengers braved the muddy 60-yard (55m) trench in Llanwrtyd Wells, Powys, on Monday.
Royal Marine Philip John, 19, from Bridgend, won three successive titles between 2002 and 2004.
He said the winning formula was simple: "put your head down and kick as hard you can".
Mr John, who was unable to take part in this year's championship, said participants needed "guts and determination" too.
He also admitted that the bog "stank" and was "freezing", but he joked that as a Marine he was now getting paid to dive into them.
Official championship rules state that entrants must not use conventional swimming strokes but instead rely on flipper power only.
Mr John, who holds the world record of 1min 35sec for completing two lengths of the bog, said: "There's no real secret to it, you just put your head down and kick as hard as you can.
"And don't worry if some bog water slips down the snorkel into your mouth.
"You need brute force and guts and determination. It helped because I did a lot of swimming, but you aren't allowed to use conventional swimming strokes."
The 2006 event attracted a crowd of about 100 to watch competitors brave the muddy trench.
Llanwrtyd Wells is no stranger to bizarre sports.
As well as bog snorkelling, each June, the town stages a man versus horse race.